We discuss the application of phase sensitive fiber optical parametric devices for the regenerative processing of high baud rate optical signals. We present recent advances in phase-sensitive amplification technology and its application to the regeneration of phase-encoded signals. By combining four wave mixing based parametric effects in highly nonlinear optical fibers and injection locking assisted synchronisation of multiple coherent lasers, we demonstrate how it possible to derive phase regeneration in signals with more than two levels of phase encoding

Kakande, J.

Parmigiani, F.

Petropoulos, P.

Richardson, D.J.

Slavík, R.

English

Southampton (e-Prints Soton)

Phase Sensitive Amplifiers for Regeneration of Phase Encoded Optical Signal Formats  Joseph Kakande*, Francesca Parmigiani, Radan Slavík, Periklis Petropoulos, and David J. Richardson  Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK Tel: +44 (0)23 8059 4521    , Fax: (0)23 8059 3142, e-mail: jkk@orc.soton.ac.uk ABSTRACT We discuss the application of phase sensitive fiber optical parametric devices for the regenerative processing of high baud rate optical signals. We present recent advances in phase-sensitive amplification technology and its application to the regeneration of phase-encoded signals. By combining four wave mixing based parametric effects in highly nonlinear optical fibers and injection locking assisted synchronisation of multiple coherent lasers, we demonstrate how it possible to derive phase regeneration in signals with more than two levels of phase encoding.   Keywords: All-optical signal processing, optical regeneration, phase sensitive amplification, fiber optical parametric amplification.  1. INTRODUCTION When transmitted over very long distances, the requirement to maintain signal power at a level sufficient to overcome  transmission  losses  means  that  nonlinear  impairments  start  to  be  introduced.  These  degradations normally involve cross or self phase  modulation between the single channel data signal, any in-band noise introduced from quantum loss and optical amplification, as well as any neighbouring co-propagating optical data channels.  These nonlinear phase impairments can be minimal for short optical links, but rapidly become the major limiting factor for long haul phase encoded optical channels [1]. When phase noise increases to drastic levels,  signal  regeneration  is  required  to  eliminate  as  much  of  the  accumulated  noise  as  possible  prior  re-transmission. Signal regeneration is currently performed in commercial systems using optical-electrical-optical (O-E-O) conversion. While OEO regeneration, particularly when assisted by forward error correction (FEC), is capable of perfectly restoring the original signal, such regenerators require a full receiver and transmitter per optical channel, implying cost and power penalties, which become more of an issue as symbol rates rise. With this in mind, all-optical solutions remain very attractive. The use of optical nonlinearities in fibers has long been studied for optical regeneration over the last few decades, with demonstrated examples including the Mamyshev-regenerator (which employs self-phase modulation in an optical fiber followed by spectral filtering in order to regenerate intensity modulated signals [2]. However, the regeneration of phase-encoded signals is becoming ever more  of  interest  in  modern  high-speed  transmission  systems,  where  the  use  of  such  modulation  formats  is favored for achieving high spectral efficiency. Phase sensitive (PS) optical parametric processes allow direct measurement and manipulation of the optical phase [3], and as such allow all-optical phase regeneration. In the work  described  here,  we  demonstrate  the  regeneration  of  phase-encoded  signals  by  making  use  of  phase-sensitive amplification (PSA) based on parametric processes in highly nonlinear optical fibers (HNLFs). Suitable combinations of such processes allow even the regeneration of signals with multiple levels of phase encoding, by effectively quantizing the phase at the output of the system to a corresponding number of levels [4].  2. CONCEPT AND EXPERIMENT  (a)  (b)  Figure 1: Phase-insensitive (PI) versus phase-sensitive (PS) amplification (taken from [3]). Figure 1 highlights the difference between conventional (PI) and phase-sensitive (PS). In a PI amplifier such as an  erbium  doped  fiber  amplifier  (EDFA),  both  in-phase  and  quadrature  components  of  a  signal  experience identical amplification irrespective of input phase. This PI gain is accompanied by addition to the signal of a certain amount of quantum noise, related to the 3 dB quantum noise figure of PIs [5]. In a PS amplifier, it is possible to selectively amplify only one of the input signal quadratures while de-amplifying the other. This process allows not only  noiseless amplification, but also phase regeneration provided that the quadrature being amplified matches that bearing the data modulation, with the noise confined to the de-amplified quadrature.  DataPump 1(cw)HNLF 1AddPump 1DataPump 1DataIdler(DE)MUXPZTAttenuatorMirrorPump 1Slave laser(Injection locked)IdlerPump 2DataPump 1 Pump 2DataEDFAHNLF 2PSAOUTDataPump 2 Pump 1DEMUXDataPump 1Pump 2Detector231-1  3140 or 56 Gbit/sBroadbandnoiseWhite noise40 GHzPhasemodulatorRegenerator(black-box)INFeedback controlDPSKTransmitterDifferentpathsboxMirrorPol. controller Figure 2: Black-box binary phase regenerator setup. HNLF- Highly Nonlinear Fiber. PZT – Piezo-electric fiber stretcher. EDFA – erbium doped fiber amplifier. DEMUX – optical demultiplexer.     Figure  2  shows  the  setup  for  a  black  box  phase  regeneration  experiment.  A  40  Gbit/s  DPSK  signal  was generated by modulating a Discrete mode semiconductor laser (Eblana Photonics). An electro-optic modulator was used to apply Gaussian phase noise to emulate nonlinear phase noise in transmission links.  The signal was combined with a continuous wave (CW) local pump (‘Pump 1’, its frequency 200 GHz away from the data carrier  frequency)  using  an  add-multiplexer  giving  insertion  loss  ~0.5  dB.  They  were  then  mixed  in  a polarization  maintaining  HNLF  (OFS  Fitel,  Denmark).  The  length,  dispersion,  nonlinear  coefficient  and insertion loss of this HNLF were 300 m, 1.3 ps/nm/km, 10.5 /W/km and 0.9 dB respectively. The semiconductor slave laser was then injection locked to the modulation stripped [3] four wave mixing idler generated in HNLF 1, eliminating all the amplitude noise on the idler, as well as high frequency phase noise. This locally generated beam now serves as pump 2 for the PSA. The data and pumps are launched into the PSA which consisted of a high-power EDFA (total power of 29 dBm) and a 180-m sample of a high-SBS-threshold HNLF (OFS Fitel, Denmark)  [6].  HNLF2  had  an  alumino-silicate  core  and  had  a  linear  strain  gradient  along  its  length.  The dispersion, polarization mode dispersion, nonlinear coefficient and attenuation of the fiber were -0.17 ps/nm/km, 0.35 ps/km-0.5, 7.5 /W/km and 15 dB/km, respectively. The relative powers of the pumps and signals  were adjusted for optimal regeneration performance (using the attenuator and the bias current of the slave laser in pump paths 1 and 2 respectively). The PZT-fiber stretcher was used as the actuator for a slow feedback loop to compensate for any relative phase drifts picked up due to thermal effects or acoustic pickup experience between HNLF 1 and HNLF 2.                 Figure 3: BER curves at the input (open) and output (full) of the regenerator for no added phase noise (black) and two levels of phase noise added (red): (a, left figure) noise level 1, (b, right figure) noise level: 2   Figure 3 shows the results of bit error ratio (BER) measurements at 40 Gbit/s at the input and output of the regenerator for various levels of phase noise. In Fig. 3a the level of added noise is such that the regenerator is fully capable of restoring the data fidelity despite the fact that the noisy data exhibited an error floor close to 10-7 . In Fig. 3b we show the same results for a greater amount of applied phase noise. In this case, the regenerator fully restores the signal fidelity up to BER 10-4 which is well within the FEC limit. In comparison with the input signal, we obtain a power penalty improvement of 9 dB at 10-4. The noticeable BER improvement shown in Fig. 3  is  due  to  a  correctable  class  of  errors  specific  to  a  ‘differential’  receiver  –  e.g.,  given  two  consecutive transmitted  bits with original phase 0˚which after transmission pick up phase noise such that they are now +45˚  and -45˚ relative to the signal carrier,  the demodulated signal  after a delay line interferometer would represent a (differential) phase  error of 90˚   making it impossible to correctly detect the original encoded message clearly resulting in an  erroneous detection. This can be fully remedied by the regenerator which acts on the absolute phase of each symbol.  3. MULTILEVEL PHASE REGENERATION   Multi-level phase regeneration requires a stair-case phase transfer function. Here, we demonstrate an example with M=4, conventionally known as quadrature phase shift keying (QPSK). QPSK is particularly relevant as it is the modulation format of choice for 100 Gbit/s Optical Ethernet. We can synthesize an M-level phase staircase by coherently adding to the input signal a phase conjugated harmonic (generated by a cascaded FWM process) bearing a temporal phase modulation M-1 times that on the signal, analogous to Fourier synthesis of complex signals [4].  The resulting waveform has its phase quantized to M-levels and an amplitude response of periodicity π/M. For QPSK, phase harmonics of the signal are generated using four wave mixing with a CW pump first, Fig. 2. The 2nd harmonics is subsequently dropped and the 4th harmonic that is CW (as any phase level presented in QPSK of 0, 90, 180, 270 deg multiplied by four gives zero phase level) was used for phase locking of  pump 2 in the next stage that was in fact a 2-pump non-degenerate PSA, Fig. 4.    (a)   (b)                   -40 -20 0 20 40-20-100Differential Phase Error (deg)Probability (dB)  (c)                          (d)                              Figure 4: (a) - Regeneration in two steps: (1) Nonlinear generation of phase harmonics FWM followed by pump recovery, (2) Parametric dual pump non-degenerate PSA. (b)-  Experimental QPSK differential (symbol-to-symbol) phase error distribution, red lines at regenerator output and black dotted line is signal without any added noise. (c), (d) - QPSK input and output constellations respectively.       As an example, a PS mixer operating on this concept was experimentally tested with a 10 Gbaud QPSK input signal. The QPSK signal was degraded using both added Gaussian optical noise and nonlinear phase noise emulated with a phase modulator. Following the PS mixer, the phase and amplitude excursions are significantly reduced.  The differential phase error distribution (symbol-to-symbol phase difference) is shown Fig. 4(b). As can  be  seen,  the  probability  of  differential  decoded  digital  errors  (symbols  with  differential  error  >45° )  is reduced significantly. The signal constellations before and after the regenerator are also shown in Fig. 4(c) and 4(d) respectively.   4. CONCLUSIONS   In  conclusion,  PSAs  now  allow  for  the  black-box  regeneration  of  various  phase  modulation  formats  by exploiting a new class of optical signal processing functions, advances in highly nonlinear fiber technology as well  improved  phase  locking  of  high  power  coherent  optical  pumps.  They  can  provide  significant  BER improvements in an asynchronously operated device, providing a degree of  flexibility and reconfigurability suited to future transparent optical networks.  ACKNOWLEDGEMENTS The authors are grateful to Dr L. Gruner-Nielsen (OFS, Denmark) and Dr R. Phelan (Eblana Photonics, Ireland) for providing respectively the HNLFs and narrow linewidth sources used in these experiments. This research has received funding from the European Communities Seventh Framework Programme FP/2007-2013 under grant agreements 224547 (PHASORS). REFERENCES [1]  J. P. Gordon, and L. F. Mollenauer, “Phase noise in photonic communications systems using linear amplifiers,” Opt. Lett. 15(23), 1351–1353 (1990). [2]  P. V. Mamyshev, "All-optical data regeneration based on self-phase modulation effect," in ECOC'98, Madrid, pp. 475-476, 1998. [3]  R. Slavik et al, "All-optical phase and amplitude regenerator for next-generation telecommunications systems," Nature Photonics, vol. 4, pp. 690-695, 2010. [4]  J. Kakande et al, “First demonstration of all-optical QPSK signal regeneration in a novel multi-format phase sensitive amplifier,” ECOC 2010 Turin 19-23, Italy, PD 3.3,  Sep 2010. [5]  C. M. Caves, “Quantum limits on noise in linear amplifiers”,  Phys. Rev. D 26, 1817 (1982). [6]  L. Grüner-Nielsen et al,  "Silica-based highly nonlinear fibers with a high SBS threshold", IEEE Photonics Society 2011 Winter Topical meeting, Paper MD4.2 (2011).  

All-optical data regeneration based on self-phase modulation effect,&quot;

All-optical phase and amplitude regenerator for next-generation telecommunications systems,&quot;

First demonstration of all-optical QPSK signal regeneration in a novel multi-format phase sensitive amplifier,” ECOC

Phase noise in photonic communications systems using linear amplifiers,”

Quantum limits on noise in linear amplifiers”,

Silica-based highly nonlinear fibers with a high SBS threshold&quot;,

Phase sensitive amplifiers for regeneration of phase encoded optical signal formats

Phase sensitive amplifiers for regeneration of phase encoded optical signal formats

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